Putilin, et al in 1991 successfully synthesized and studied a family of complex mercury-containing copper oxides, HgBa.sub.2 RCu.sub.2 O.sub.7 (R=La, Nd, Eu, Gd, Dy, Y). See Putilin, et al: "New Complex Copper Oxides: HgBa.sub.2 RCu.sub.2 O.sub.7 (R=La, Nd, Eu, Gd, Dy, Y)," Mat. Res. Bull. Vol. 26, pp. 1299-1307 (1991). This family of oxides were considered as first examples of Hg-containing complex copper oxides with intergrowth structures ("1212" structural type. However, it was reported that magnetic measurements performed on these compounds down to 12 K. did not show noticeable diamagnetic effects. No other superconducting characteristic was observed with this family of compounds.
In 1992, R. S. Liu, one of the co-inventors of the present invention, discovered, along his coworkers, that bulk superconductivity of up to 92 K. can be achieved in the mercury-containing curprate, (Tl.sub.0.5 Hg.sub.0.5)Sr.sub.2 (Ca.sub.1-x Y.sub.x)Cu.sub.2 O.sub.7-.delta.. See R. S. Liu, S. F. Hu, D. A. Jefferson, P. P. Edwards, and P. D. Hunnyball: "A New 92 K. High-T.sub.c Superconductor," PHYSICA C, vol. 205, pp. 206-211 (1993). It is to be noted that the bulk superconductivity of up to 92 K. was measured and reported based on the temperature of onset diamagnetism. The zero-resistance temperature, T.sub.c(zero), of the mercury-containing cuprate synthesized by Liu et al was substantially lower, at 76 K.
Essentially concurrently with the discovery of Liu et al discussed above, S. F. Hu, D. A. Jefferson, R. S. Liu (a co-inventor of the present invention), and P. P. Edwards in an article entitled: "Superconductivity up to 90 K. in a New Family of the (Pb, Hg)Sr.sub.2 (Ca, Y)Cu.sub.2 O.sub.7 System," Journal of the Solid State Chemistry, vol. 103 pp. 280-286 (1993), disclosed a family of superconductors in the (Pb.sub.0.5 Hg.sub.0.5)Sr.sub.2 (Ca.sub.1-x Y.sub.x)Cu.sub.2 O.sub.7-.delta. system for the composition range of 0.5.gtoreq.x.ltoreq.0.2, with a maximum T.sub.c of 90 K. However, the 90 K. "T.sub.c " reported in the Hu article was referring to the "midpoint" superconducting temperature; its zero-resistance temperature, T.sub.c(zero), was also substantially lower, at 77 K.
In an article entitled: "Superconductivity Above 130 K. in the Hg-Ba-Ca-Cu-O System," by A Schilling, M. Cantoni, J. D. Guo, and H. R. Ott, Nature, vol. 363, pp 56-58, it was disclosed a superconducting material containing HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.1+x (with three CuO.sub.2 layers per unit cell), HgBa.sub.2 CaCu.sub.2 O.sub.6+x (with two CuO.sub.2 layers per unit cell), and an ordered superstructure comprising a defined sequence of the unit cells of these phases. In that article, Schilling and the co-authors reported magnetic and resistivity measurements conducted on these materials confirmed a maximum transition temperature of about 133 K. However, the Hg-Ba-Ca-Cu-O superconducting system disclosed by Schilling et al involves a very complicated synthesis procedure, which would lead to low purity of the final product and difficulties in attempts to produce commercial quantities. Furthermore, the Hg-Ba-Ca-Cu-O material can be easily hydrolyzed when exposed to the atmosphere. This weakness could greatly undermine its practical usefulness.
Comparing the Hg-containing Sr-based phases of (Tl.sub.0.5 Hg.sub.0.5)Sr.sub.2 (Ca.sub.1-x Y.sub.x)Cu.sub.2 O.sub.7-.delta. and (Pb.sub.0.5 Hg.sub.0.5)Sr.sub.2 (Ca.sub.1-x Y.sub.x)Cu.sub.2 O.sub.7-.delta. with the Hg-containing Ba-based Hg-Ba-Ca-Cu-O phase, the former can be prepared from its constituent metal oxides in air; whereas, the later must be prepared in a strictly controlled condition, which typically involves a moisture-free glove box. Furthermore, after synthesis, the Hg-containing Sr-based phases of (Tl.sub.0.5 Hg.sub.0.5)Sr.sub.2 (Ca.sub.1-x Y.sub.x)Cu.sub.2 O.sub.7-.delta. and (Pb.sub.0.5 Hg.sub.0.5)Sr.sub.2 (Ca.sub.1-x Y.sub.x)Cu.sub.2 O.sub.7-.delta. are also more stable than the Hg-containing Ba-based Hg-Ba-Ca-Cu-O phase, and can be stored in ambient or open air. Therefore, the Hg-containing Sr-based superconductor material has better potential for practical applications, and can be more easily manufactured in large production quantities than the Hg-containing Ba-based type superconductor material.
In 1994, Hahakura, S., Shimoyama, J., Shiino, O., and Kishio, K., disclosed in an article entitled: "New Barium-Free Mercury-Based High-T.sub.c Superconductors (Hg, Mo)Sr.sub.2 (Ca, Y).sub.n-1 Cu.sub.n O.sub.y and HgSr.sub.2 (Ca, Y).sub.n-1 (Cu, Re).sub.n O.sub.y (n=1 and 2)," a Ba-free Hg based superconductor having good stability under open air. However, the zero resistance temperature T.sub.c(zero) was measured to be only at 58 K., substantially higher than all the superconductor discussed above.